The Holmes Stereoscope was the most popular photo viewer of the 19th century.  It was invented patent-free by Oliver Wendell Holmes Sr, father of the Associate Justice of the Supreme Court Oliver Wendell Holmes, Jr. This viewer was used by Lula Hillhouse of Anderson in the 19th century.
Slide for the stereoscope
Slide for the stereoscope
Artist's rendering of the effect as seen through the stereoscope
Artist's rendering of the effect as seen through the stereoscope